This invention relates to the target structure of a photoconductive image pickup tube and more particularly to the target structure of a photoconductive image pickup tube having a heterogeneous junction capable of manifesting improved operating characteristics and a method of manufacturing such a target structure.
As an image pickup tube including a target which utilizes a non-crystalline photoconductive film, a vidicon has been known which includes an ohmic junction utilizing a film of antimony trisulfide.
Recently, an image pickup tube including a photoconductive target which utilizes a non-crystalline photoconductive film wherein use is made of a heterogeneous junction between a P-type photoconductive film containing selenium and an intensifier such as tellurium and an N-type conductive film such as Nesa film has been proposed.
The image pickup tube of this type is characterized in that it has a wide range of spectrum sensitivity, a fast response time, low dark current and a high resolution and that it is easy to manufacture.
Typically, the target structure of the image pickup tube having these characteristics is constructed such that a transparent conductive film consisting essentially of indium oxide or stannic oxide having N-type conductivity is coated on the rear surface of a glass substrate or a transparent glass window that transmits the incident rays to the image pickup tube and that a P-type photoconductive film comprising selenium, less than 30 atomic % of tellurium and less than 30 atomic % of arsenic, for example, a P-type photoconductive film comprising a mixture of a first photoconductive substance consisting of selenium and 40 atomic % of tellurium and a second photoconductive substance consisting of selenium and 10 atomic % of arsenic is deposited on the rear surface of the N-type transparent conductive film through a heterogeneous junction surface. According to another type, an N-type transparent semiconductive film is formed on the rear surface of said N-type transparent conductive film by the vapour deposition of cadmium selenide, cadmium sulfide, zinc sulfide, gallium arsenic, germanium or silicon and said P-type photoconductive film is formed on the rear surface of the N-type transparent semiconductive film through a heterogeneous junction surface. Furthermore, for the purpose of improving the landing characteristic of an electron beam emitted from an electron beam emitting device on the photoconductive film a porous film of antimony trisulfide (Sb.sub.2 S.sub.3) is formed on the rear surface of the P-type photoconductive film. In these cases, as will be described later with reference to the accompanying drawings the tellurium in the first photoconductive substance presents throughout the thickness of the P-type photoconductive film and the concentration of the tellurium increases substantially continuously from the heterogeneous junction surface whereas the concentration of the arsenic in the second photoconductive substance is substantially uniform from the heterogeneous junction surface to the P-type photoconductive film and throughout the thickness thereof. Although the purpose of tellurium is to improve the light absorption characteristic it has a larger tendency of crystallization under heat than selenium, so that much quantity of tellurium enhances the crystallization of the P-type photoconductive film thus causing local decrease in the film resistance. This results in the defects of the picture picked up in the form of white spots, thus greatly decreasing the quality of the picture.
With this construction, the region in which the concentration of tellurium is high and hence having an extremely low specific resistance is located close to the heterogeneous junction surface so that the heterogeneous junction is deteriorated and the initial dark current characteristic is greatly impaired. Where the target is stored or left standstill in atmosphere at a temperature of higher than 60.degree. C the heterogeneous junction surface is deteriorated to increase the dark current due to a slight diffusion of tellurium. Such variation in the dark current characteristic causes a poor colour balance of the picture picked up by the image pickup tube thus degrading the quality of the picture.